A window envelope is an envelope with one or more openings of any shape, typically rectangular, that allows examination of information, usually a barcode, name and address, printed on a limited area of matter disposed within the envelope. The opening or openings may be sealed or closed with a window patch composed of a film that allows the printed information to be viewed by individuals as well as by optical character readers such as USPS automatic mail sorting systems.
Patching (during envelope manufacturing) is the process in which window envelope film is cut into patches of a desired length and width, gummed around the edges, and applied to the window envelope. Alternatively, the edges of the window opening or openings may be gummed instead of the edges of the window patch.
The patching process requires the use of a vacuum drum to ensure proper positioning of the film. As the film passes over the surface of the vacuum drum there exists a disparity in the speed of the passing film and the angular velocity of the drum. This disparity of speed results in the abrasion and scratching of the film. Scratching of the film may also occur before and after the patching process for example in film roll handling, loading, and unwinding prior to the patching process and envelope handling post the patching process. The scratches can range in severity and are visible to the human eye and thus can make the window envelope patches less aesthetically pleasing. It is an object of the present invention to produce an improved scratch resistant polystyrene-based film that can be used as a window envelope film to produce a window envelope patch having few or less visible scratches.
According to the present invention, there is now provided a film that is more scratch resistant when used as window envelope film than window envelope film currently used. Advantageously, films of the present invention, when subjected to scratch testing, exhibit scratch resistance in two ways. First, upon visual inspection, film of the present invention has significantly fewer visible scratches than the “control” film. Second, when the film of the present invention is subjected to scratch testing, its haze value does not increase near as much as the “control” film that is subjected to the same scratch testing.
Current window envelopes include window patches fabricated from commercial biaxially oriented films that typically comprise general purpose polystyrene (GPPS)/high impact polystyrene (HIPS) resin blends with HIPS resin usually constituting less than (<) 2.5 weight percent rubber based on weight of polymers in the blend used to make the window patch. This usually corresponds to less than about 40 weight percent (wt %) HIPS in the blend, based upon blend weight.
U.S. Pat. No. 5,009,953 to Foster et al. discloses a window envelope having a non-opaque plastic window patch formed of a film comprising polystyrene and from about 0.1 to about 3.0 wt % of one or more particulate anti-flecking agents having a number average particle size of from about 0.1 micrometer (μm) to about 10.0 μm.
Published patent application, WO 99/43740, to Liang discloses a window envelope film having improved scratch resistance by utilizing a nylon copolymer, a styrenic block copolymer, and optionally inorganic particulates. The nylon would preferably have a melt temperature less than 150° C.